Students across the UK could be left unable to graduate after academics announced plans for a marking boycott in an increasingly bitter row over pay.

In a further escalation of its industrial action, the University and College Union (UCU) said it has given the green light for its members to refuse to mark exam papers and assessments if the dispute is not resolved in the next two months.

The move, described by the union as its "ultimate sanction", could mean that students are left without the final results they need to gain their degree, and therefore would be unable to graduate.

The dispute focuses on a 1% pay rise offered to university staff, which UCU insists means that their members have faced a 13% pay cut in real terms since October 2008.

UCU, along with a number of other unions, has already staged three one-day national walkouts as part of their campaign, and has also held a series of two-hour strikes aimed at disrupting teaching.

University employers have previously expressed disappointment at the UCU's action, and maintained that the strikes were not well-supported.

UCU said that its marking boycott - the first the union has conducted since 2006 - will begin on April 28 if a pay deal is not reached.

The union said it hoped that employers would sit down with them to work out a pay settlement in the next few weeks, adding it had been frustrated with employers' stance on the issue.

General secretary Sally Hunt said: "A marking boycott is the ultimate sanction, but an avoidable one if the employers would negotiate with us over pay. No member I have spoken to wishes to see this dispute escalate, but in the continued absence of meaningful negotiations from the employers, we are left with no alternative.

"I fail to see how any university can claim to have students' best interests at heart if it is not pushing for talks with the union to resolve this dispute. Even now the timetable we have set provides a generous window of opportunity for the employers to address our just demands, which we, and students, hope they take."

She added: " The strong support for our action so far demonstrates how angry staff are at the hypocrisy over pay in our universities. The employers cannot plead poverty when it comes to staff pay and then award enormous rises to a handful at the top."

A spokesman for the Universities and Colleges Employers Association (UCEA) said: "Higher education (HE) institutions will certainly be disappointed that the UCU is still threatening a marking boycott, as this is action that is once again aimed directly at students' education.

"Both UCU and HE institutions are well aware that strike support has dwindled still further in recent weeks, with the overwhelming majority of staff not taking part and having no wish to disrupt students' education.

"It is quite extraordinary for the UCU to be planning yet more action over last year's pay uplifts with the 2014-15 pay negotiations due start in March, a full month prior to the start of their threatened marking boycott. Everyone is aware that last year's pay uplifts averaging 3% have already been paid.

"The employers were willing to explore whether both sides could engage in positive dialogue about how they could approach the 2014-15 pay round, rather than continue to dispute last year's pay claim.

"Clearly HE institutions will be affirming their policies for withholding full pay for any staff who follow such a damaging course of industrial action aimed at their students' education."